The LTE system is a mobile communication system that has evolved from a UMTS system, and the standard has been established by 3rd Generation Partnership Project (3GPP), which is an international standardization organization.
FIG. 1 is a view illustrating the network architecture of an LTE system, which is a mobile communication system to which the related art and the present invention are applied.
As illustrated in FIG. 1, the LTE system architecture can be roughly classified into an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and an Evolved Packet Core (EPC). The E-UTRAN may include a user equipment (UE) and an Evolved NodeB (eNB, base station), wherein the connection between UE-eNB is called a Uu interface, and the connection between eNB-eNB is called an X2 interface. The EPC may include a Mobility Management Entity (MME) performing a control-plane function and a Serving Gateway (S-GW) performing a user-plane function, wherein the connection between eNB-MME is called an S1-MME interface, and the connection between eNB-S-GW is called an S1-U interface, and both connections may be commonly called an S1 interface.
A radio interface protocol is defined in the Uu interface which is a radio section, wherein the radio interface protocol is horizontally comprised of a physical layer, a data link layer, a network layer, and vertically classified into a user plane (U-plane) for user data transmission and a control plane (C-plane) for signaling transfer. Such a radio interface protocol can be typically classified into L1 (first layer) including a PHY layer which is a physical layer, L2 (second layer) including MAC/RLC/PDCP layers, and L3 (third layer) including a RRC layer as illustrated in FIGS. 2 and 3. Those layers exist as a pair in the UE and E-UTRAN, thereby performing data transmission of the Uu interface.
FIGS. 2 and 3 are exemplary views illustrating the control plane and user plane architecture of a radio interface protocol between UE and E-UTRAN in an LTE system, which is a mobile communication system to which the related art and the present invention are applied.
The physical layer (PHY) which is a first layer provides information transfer services to the upper layers using a physical channel. The PHY layer is connected to the upper Medium Access Control (MAC) layer through a transport channel, and data between the MAC layer and the PHY layer is transferred through the transport channel. At this time, the transport channel is roughly divided into a dedicated transport channel and a common transport channel based on whether or not the channel is shared. Furthermore, data is transferred between different PHY layers, i.e., between PHY layers at the transmitter and receiver sides.
Various layers exist in the second layer. First, the Medium Access Control (MAC) layer serves to map various logical channels to various transport channels, and also performs a logical channel multiplexing for mapping several logical channels to one transport channel. The MAC layer is connected to an upper Radio Link Control (RLC) layer through a logical channel, and the logical channel is roughly divided into a control channel for transmitting control plane information and a traffic channel for transmitting user plane information according to the type of information to be transmitted.
The Radio Link Control (RLC) layer of the second layer manages segmentation and concatenation of data received from an upper layer to appropriately adjusts a data size such that a lower layer can send data to a radio section. Also, the RLC layer provides three operation modes such as a transparent mode (TM), an un-acknowledged mode (UM) and an acknowledged mode (AM) so as to guarantee various quality of services (QoS) required by each radio bearer (RB). In particular, AM RLC performs a retransmission function through an automatic repeat and request (ARQ) function for reliable data transmission.
A Packet Data Convergence Protocol (PDCP) layer of the second layer performs a header compression function for reducing the size of an IP packet header, which is relatively large in size and contains unnecessary control information to efficiently transmit IP packets, such as IPv4 or IPv6, over a radio section with a relatively small bandwidth. Due to this, information only required from the header portion of data is transmitted, thereby serving to increase the transmission efficiency of the radio section. In addition, in the LTE system, the PDCP layer performs a security function, which includes ciphering for preventing the third person's data wiretapping and integrity protection for preventing the third person's data manipulation.
A radio resource control (RRC) layer located at the uppermost portion of the third layer is only defined in the control plane. The RRC layer performs a role of controlling logical channels, transport channels and physical channels in relation to configuration, re-configuration, and release of Radio Bearers (RBs). Here, the RB denotes a logical path provided by the first and the second layers for transferring data between the UE and the UTRAN. In general, the establishment of the RB refers to a process of stipulating the characteristics of protocol layers and channels required for providing a specific service, and setting each of the detailed parameter and operation method thereof. The RB is divided into a signaling RB (SRB) and a data RB (DRB), wherein the SRB is used as a path for transmitting RRC messages in the C-plane while the DRB is used as a path for transmitting user data in the U-plane.
Hereinafter, a PDCP entity will be described in detail. The PDCP entity is upwardly connected to a RRC layer or user application, and downwardly connected to a RLC layer, and the detailed architecture thereof is as follows. FIG. 4 is an exemplary view illustrating the architecture of a PDCP entity. The blocks illustrated in FIG. 4 are functional blocks, and may differ from actual implementation.
One PDCP entity may include a transmitter side and a receiver side as illustrated in FIG. 4. The transmitter side on the left performs a role of configuring SDU received from the upper layer or control information generated by the PDCP entity itself as PDU to transmit to a receiver side of the peer PDCP entity, and the receiver side on the right performs a role of extracting PDCP SDU or control information from the PDCP PDU received from a transmitter side of the peer PDCP entity.
As described above, there are two kinds of PDUs, data PDU and control PDU, which are generated by the transmitter side of the PDCP entity. First, PDCP Data PDU is a data block made in PDCP by processing SDU received from the upper layer, and PDCP Control PDU is a data block generated by PDCP itself for the PDCP to transfer control information to the peer entity.
The PDCP Data PDU is generated in RB of both the user plane and control plane, and some of the PDCP functions are selectively applied according to the used plane. In other words, a header compression function is applied only to U-plane data, and an integrity protection function within the security function is applied only to C-plane data. The security function may also include a ciphering function for maintaining the security of data in addition to the integrity protection function thereof, and the ciphering function is applied to both U-plane and C-plane data.
The PDCP Control PDU is generated only in U-plane RB, and may include roughly two types, such as a PDCP status report for informing a transmitter side of the situation of a PDCP reception buffer, and a header compression (HC) feedback packet for informing a header compressor of the situation of a header decompressor.
The data processing procedure performed by the PDCP layer of the transmitting side will be described as follows.
1. The PDCP layer of the transmitting side receives PDCP SDUs and stores the received PDCP SDUs in a transmission buffer. Then, the PDCP layer allocates a sequence number to each PDCP SDU.
2. If the established RB is that of the user plane, i.e., DRB, the PDCP layer performs header compression for the PDCP SDUs.
3. If the established RB is that of the control plane, i.e., SRB, the PDCP layer performs integrity protection for the PDCP SDUs.
4. A data block generated by the result of the procedure 2 or 3 is ciphered.
5. The PDCP layer fixes a proper header to the ciphered data block to constitute PDCP PDU, and then transfers the constituted PDCP PDU to the RLC layer.
The data processing procedure performed by the PDCP layer of the receiving side will be described as follows.
1. The PDCP layer of the receiving side removes a header from the received PDCP PDU.
2. The PDCP layer deciphers the PDCP PDU from which the header has been removed.
3. If the established RB is that of the user plane, i.e., DRB, the PDCP layer performs header decompression for the deciphered PDCP PDU.
4. If the established RB is that of the control plane, i.e., SRB, the PDCP layer performs integrity verification for the deciphered PDCP PDU.
5. A data block (i.e., PDCP SDU) generated by the result of the procedure 3 or 4 is transferred to the upper layer. If the established RB is that of the user plane, i.e., DRB, the PDCP layer stores the data block in a receiving buffer as occasion demands and performs reordering for the data block. Then, the PDCP layer transfers the resultant data to the upper layer.
Here, if the established RB is a DRB using a RLC AM (Acknowledged Mode), the reordering function should be performed. The reason for the performance of the reordering function is that the DRB using the RLC AM usually transmits error sensitive data traffic.
The security function has two kinds of functions, i.e., ciphering and integrity protection. A code varied depending on each packet is generated by these two functions, and ciphering and integrity check are performed for original data using the generated code.
The code varied depending on each packet is generated using a PDCP sequence number (SN) and added to each PDCP PDU header. For example, the code varied depending on each packet is generated using COUNT which includes PDCP SN. The COUNT has a length of 32 bits, of which the least significant bit (LSB) has a PDCP SN and the most significant bit (MSB) has a hyper frame number (HFN). The PDCP SN has a length of 5 bits, 7 bits, or 12 bits depending on a radio bearer (RB). Accordingly, the HFN has a length of 27 bits, 25 bits or 20 bits.
FIG. 5 is a diagram illustrating an example of a method of performing ciphering in a PDCP layer. A PDCP layer of a transmitting side generates ciphered data by covering original data with a MASK. The MASK is a code varied for each of the aforementioned packets. Covering original data with a MASK means that XOR operation for each bit is performed for the original data with respect to MASK. A PDCP layer of a receiving side, which has received the ciphered data, deciphers the original data by again covering the original data with a MASK. The MASK has 32 bits and is generated from several input parameters. In particular, in order to generate different values for respective packets, COUNT is generated using PDCP SN varied depending on PDCP PDU. The COUNT is used as one of MASK generation input parameters. In addition to the COUNT, examples of the MASK generation input parameters include ID value (bearer of FIG. 5) of a corresponding RB, Direction having an uplink or downlink value, and a ciphering key (CK) exchanged between a user equipment and a network during RB establishment.
FIG. 6 is a diagram illustrating an example of a method of performing integrity protection in a PDCP layer. Similarly to the aforementioned ciphering procedure, in an integrity protection procedure, parameters, such as COUNT based on PDCP SN, bearer which is ID value of RB, Direction having an uplink or downlink value, and integrity protection key (IK) exchanged between a user equipment and a network during RB establishment, are used. A specific code, i.e., MAC-I (Message Authentication Code-Integrity) is generated using the above parameters. The integrity protection procedure is different from the aforementioned ciphering procedure in that the generated MAC-I is added to PDCP PDU not undergoing XOR operation with original data. The PDCP layer of the receiving side, which has received the MAC-I, generates XMAC-I using the same input parameter as that used in the PDCP layer of the transmitting side. Afterwards, XMAC-I is compared with MAC-I, and if two values are equal to each other, it is determined that the data have integrity. If not so, it is determined that the data have been changed.
Hereinafter, a Long-Term Evolution Advanced (LTE-A) system will be described. The LTE-A system is a system that has been developed from an LTE system to meet IMT-Advanced conditions, which are the fourth generation mobile communication conditions recommended by the International Telecommunication Union-Radiocommunication Sector (ITU-R). At present, the LTE-A system standard is actively under development by 3GPP that has developed the LTE system standard. Representative technologies newly added in the LTE-A system mi carrier aggregation technology for extending a used bandwidth to be flexibly used, and relay technology for increasing coverage, supporting group mobility, and allowing network arrangement.
Here, relay is a technology for relaying data between a user equipment (EU) and an Evolved Node B (eNB, base station). Since communication is not smoothly implemented in case where a distance between UE and eNB is far in the LTE system, it is introduced in an LTE-A system as a method of making up for the problem. A new network node, which is referred to as Relay Node (RN), is introduced between UE and eNB to perform such a relay operation, wherein the eNB for managing RN is called Donor eNB (DeNB). In addition, an interface between RN-DeNB that has been newly added due to RN is defined as an Un interface, thereby being differentiated from a Un interface that is an interface between UE and a network node. FIG. 7 illustrates such a concept of Relay Node and an Un interface.
Here, the RN serves to manage UE in behalf of the DeNB. In other words, from a standpoint of the UE, the RN is shown as DeNB, and therefore, MAC/RLC/PDCP/RRC, which is an Uu interface protocol that has been used in a conventional LTE system, is used as they are in a Uu interface between UE-RN.
From a standpoint of the DeNB, the RN may be shown as UE or shown also as eNB according to circumstances. In other words, when the RN is first accessed to the DeNB, it is accessed through random access like UE because the existence of the RN is unknown to the DeNB, but operated like eNB managing UE connected to itself after the RN is once accessed to the DeNB. Accordingly, along with the Uu interface protocol, the functions of the Un interface protocol should be also defined as in the form to which a network protocol function is also added. For the Un interface protocol, discussions as to which functions should be added or changed to each protocol layer on the basis of Uu protocols such as MAC/RLC/PDCP/RRC are still in progress in 3GPP.
The Un radio protocol and the Uu radio protocol have no difference each other, as the RN operates just like the UE. Here, the network protocol may be divided into a S1 protocol and a X2 protocol. Usually, the RN supports the S1 protocol for communicating with MME or S-GW in the UN interface, and supports the X2 protocol for communicating with other eNBs.